The Endocrine System works alongside the Nervous System to co-ordinate the functions of all the body systems. However, the authors do not wish to stray into the area of the Nervous System so what follows is a simplified description of endocrine glands and the hormones they release
Major Endocrine Glands
The primary glands that make up the endocrine system are the hypothalamus, pituitary, thyroid, parathyroid, adrenal, pineal and gonads (testes and ovaries). The pancreas, an organ often associated with the digestive system is also considered part of the endocrine system. In addition, some nonendocrine organs are known to actively secrete hormones. These include the brain, heart, lungs, kidneys, liver, thymus, skin and placenta.
Structure and Function of Adrenal Glands
The hypothalamus, found deep within the brain, directly control the pituitary gland. It is sometimes described as the coordinator of the endocrine system. When information reaching the brain indicates that changes are3 needed somewhere in the body, nerve cells in the hypothalamus secrete body chemicals that either stimulate or suppress hormone secretions from the pituitary gland. The hypothalamus is the main link between the nervous system and the endocrine system.
Located in a bony cavity just below the base of the brain is one of the endocrine systems most important parts — the pituitary gland. This is often described as the body's master gland, and it secretes several hormones that regulate the function of the other endocrine glands. Structurally, this gland is divided into two parts, the anterior and posterior lobes, each having separate functions. The anterior lobe regulates the activity of the thyroid and adrenal glands as well as the reproductive glands. It also regulates the body's growth and stimulates milk production in women who are breastfeeding. The anterior lobe also secretes endorphins, chemicals that act on the nervous system to reduce sensitivity to pain.
The thyroid glands, located in the neck, secrete hoimones in response to stimulation by thyrotropic hormone (TSH) from the pituitary gland. The thyroid secretes hormones — for example thyroxine — that regulates growth and metabolism and plays a role in brain development during childhood.
These glands are four small glands located at the four corners of the thyroid gland. The hormone they secrete, parathyroid hormone, regulates the level of calcium in the blood
The adrenal glands are located on top of the kidneys and have two distinct parts. The outer part called the adrenal cortex produces a variety of hormones called cortisteroids, which include cortisol. These hormones regulate salt and water balance in the body for stress, regulate metabolism, interact with the immune system and influence sexual function. The inner part, the adrenal medulla, produces epinephrine, also called adrenaline, which increases the blood pressure and heart rate during times of stress.
The pineal gland, also called the pineal body, is located in the middle of the brain. It secretes melatonin, a hounone that may help regulate the wake-sleep cycle. Research has shown that disturbances in the secretion of melatonin are responsible, in part, for the jet lag associated with long-distance air travel.
Gonads (Testes and ovaries)
The gonads are the reproductive components of the endocrine system. They secrete sex hormones in response to stimulation from the pituitary gland. Located in the pelvis, the female gonads (ovaries) produce eggs. They also secrete a number of female sex hormones, including estrogen and progesterone, which control development of the reproductive organs, stimulate the appearance of female secondary sex characteristics, and regulate menstruation and pregnancy located in the scrotum, the male gonads, the testes, produce sperm and also secrete a number of male sex hormones, or androgens. The androgens, the most important of which is testosterone, regulate development of the reproductive organs, stimulate male secondary sex characteristics, and stimulate muscle growth.
The pancreas is positioned in the upper abdomen, just under the stomach. The major part of the pancreas, called the exocrine pancreas, functions as an exocrine glands, secreting digestive enzymes into the GI tract. Distributed through the pancreas are clusters of endocrine cells that secrete insulin, glucagons and somastatin. These hormones all participate in regulating energy and metabolism in the body.
Physiology of the Endocrine System
Hormones from the endocrine organs are secreted directly into the bloodstream, where special proteins usually bind to them, helping to keep the hormones intact as they travel throughout the body. The proteins also act as a reservoir allowing only a small fraction of the hormone circulating in the blood to affect the target tissue. Specialized proteins in the target tissue, called receptors, bind with the hormones in the bloodstream, inducing chemical changes in response to the body's needs. Typically, only minute concentrations of a hormone are needed to achieve the desired effect.
Too much or too little hormone can be harmful to the body, so hormone levels are regulated by a feedback mechanism. Feedback works something like a household thermostat. When the heat in a house falls, the thermostat responds by switching the furnace on, and when the temperatures is too warm, the thermostat switches the furnace off. Usually, the change that a hormone produces also served to regulate that hormones secretion. This feedback system allows for tight control over hormone levels, which is essential for ideal body function. Other mechanisms may also influence feedback relationships. For example, if a person becomes ill, the adrenal glands increase the secretions of certain hormones that heal the body deal with the stress of illness. The adrenal glands work in conjunction with the pituitary gland and the brain to increase the body's tolerance of these hormones in the blood, preventing the normal feedback mechanism from decreasing secretion levels until the illness has gone.
Hormones are the 'messengers' of the body. They are chemicals released under order of the brain, via. electrical messages from the brain to different organs around the body. These small organs (or Endocrine Glands) then make and release their own specific hormones into the blood stream. These hormones tell other organs to make different chemicals or to control their blood supply.
Here is an example:...
When you brain wants to tell your heart that it needs to beat faster in order to help your muscles outrun the man chasing you, within microseconds, the nerve impulses have told the adrenal gland to release its hormone. This hormone is Adrenaline and this takes the message to the heart, as well as the other organs that are required in preparation for running.
Types of Hormone
Corticosterone - (Pronounced As: k6rtkosteron )
Steroid hormone secreted by the outer layer, or cortex, of the adrenal gland. Classed as a glucocorticoid, corticosterone helps regulate the conversion of amino acids into carbohydrates and glycogen by the liver, and helps stimulate glycogen formation in the tissues. Corticosterone is similar in structure, although somewhat less potent, than the other glucocorticoids cortisol and cortisone. It is produced in response to stimulation by the pituitary substance adrenocorticotropic hormone (ACTH).
Cortisol - (Pronounced As: kOrtis61 or hydrocortisone)
Steroid hormone that in humans is the major circulating hormone of the cortex, or outer layer, of the adrenal gland. Like cortisone, cortisol is classed as a glucocorticoid; it stimulates liver glycogen formation while it decreases the rate of glucose utilization in body cells. A main effect of cortisol is to reduce the reserves of protein in all body cells except cells of the liver and gastrointestinal tract. It also makes fatty acids available for metabolic use. Cortisol, usually referred to as hydrocortisone when used medicinally, is more potent than cortisone with respect to metabolic and anti-inflammatory effects.
Cortisone - (Pronounced As: kOrtison
A Steroid hormone whose main physiological effect is on carbohydrate metabolism. It is synthesized by cholesterol in the outer layer, or cortex of the adrenal gland under the stimulation of adrenocorticotropic hormone (ACTH). Cortisone is classed as a glucocorticoid with cortisol and corticosterone; its effects include increased glucose release from the liver, increased liver glycogen synthesis, and decreased utilization of glucose by the tissues. These actions tend to counter the effects of insulin and may aggravate or mimic diabetes in sufficiently high doses. Cortisone also exerts an effect on salt retention in the kidneys similar to that of aldosterone, although it is not as potent. The principal medical use of cortisone comes from its anti-inflammatory and anti-allergic effects; it is extremely useful in the treatment of innumerable diseases including asthma and other allergic reactions, arthritis, and various skin diseases.
Epinephrine - (Pronounced As: epnefrin )
A holinone that's important to the body's metabolism, also known as adrenaline. Epinephrine, a catecholamine, together with norepinephrine, is secreted principally by the medulla of the adrenal gland. Heightened secretion caused perhaps by fear or anger, will result in increased heart rate and the hydrolysis of glycogen to glucose. This reaction, often called the "fight or flight response, prepares the body for strenuous activity.
Oestrogen -(Pronounced As: estrjn )
Any one of a group of hormones synthesized by the reproductive organs and adrenal glands in females and, in lesser quantities, in males. The oestrogens cause the thickening of the lining of the uterus and vagina in the early phase of the ovulatory, or menstrual, cycle. The estrogens are also responsible for female secondary sex characteristics such as, in humans, pubic hair and breasts, and they affect other tissues including the genital organs, skin, hair, blood vessels, bone, and pelvic muscles. Oestrogen replacement therapy (ERT) uses synthetic Oestrogen (e.g., Premarin) to treat the physical changes of menopause, including hot flashes and vaginal dryness. It also retards the heart attack in postmenopausal women, and may have an effect on Alzheimer's disease. Oestrogens are also used to treat prostate cancer.
Gonadotropic hormone -(Pronounced As: gonadtropik or gonadotropin)
Any one of three glycoprotein hormones released by either the anterior pituitary gland or the placenta (the organ in which maternal and fetal blood exchange nutrients and waste products) that have various effects upon the ovaries and testes. This hormone travels in the bloodstream to the anterior pituitary, where it causes the release of the gonadotropic hormones. Sex hormones released from the ovaries and testes eventually reach the hypothalamus and help to regulate the hormonal cycle. Human chorionic gonadotropin (HCG), produced in the placenta, helps to maintain pregnancy once a fetus begins to develop. It appears in the urine in approximately the first week after the first missed menstrual period, and is the basis for two kinds of pregnancy tests: the `Ascheim-Zondek' and 'Friedman' tests.
Growth hormone -(or somatotropin Pronounced As: somattropn)
Glycoprotein hormone released by the anterior pituitary gland that is necessary for normal skeletal growth in humans. Evidence suggests that the secretion of human growth hormone (HGH) is regulated by the release of certain peptides by the hypothalamus of the brain.
Norepinephrine - (Pronounced As: nOre_pinefrn)
A neurotransmitter in the catecholamine family that mediates chemical communication in the sympathetic nervous system. The sympathetic nervous system functions in response to short-term stress; hence norepinephrine and epinephrine increase the heart rate as well as blood pressure. One action of norepinephrine is the relaxation of the bronchial smooth muscle to open up the air passages to the lungs. Actions such as these represent a mobilization of the body's resources in order to meet the stressful challenge-such a response is often termed the "flight or fight" syndrome.
Oxytocin -(Pronounced As: oksitosin)
Hormone released from the posterior lobe of the pituitary gland that facilitates uterine contractions and the milk-ejection reflex. Stimuli that elicit the release of oxytocin include childbirth, suckling, and coitus; the uterine contractions that result may facilitate either childbirth or the ascent of spermatozoa through the fallopian tubes. Oxytocin may also play a role in the initiation of labor. The milk-ejection response occurs only in females immediately after childbirth. The role of oxytocin in males is unknown.
Progesterone -(Pronounced As: projestron )
Female sex hormone that induces secretory changes in the lining of the uterus essential for successful implantation of a fertilized egg. A steroid, progesterone is secreted chiefly by the corpus luteum, a group of cells formed in the ovary after the follicle ruptures during the release of the egg cell. If fertilization does not take place, the secretion of progesterone decreases and menstruation occurs. If fertilization does occur, progesterone is secreted during pregnancy by the placenta and acts to prevent spontaneous abortion; the hormone also prepares the mammary glands for milk production.
Testosterone -(Pronounced As: testostron)
Principal androgen, or male sex hormone. One of the group of compounds known as anabolic steroids, testosterone is secreted by the testes but is also synthesized in small quantities in the ovaries, cortices of the adrenal glands, and placenta, usually from cholesterol. Testosterone is necessary in the fetus for the development of male external genitalia; increased levels of testosterone at puberty are responsible for further growth of male genitalia and for the development and maintenance of male secondary sex characteristics such as facial hair and voice changes. Testosterone also stimulates protein synthesis and accounts for the greater muscular development of the male.
Thyrotropin -(Pronounced As: thiratropin or thyroid-stimulating hormone (TSH).
Hormone released by the anterior pituitary gland that stimulates the thyroid gland to release thyroxine. The release of thyrotropin is triggered by the action of thyrotropin- releasing factor (TRF), a substance found in the hypothalamus of the brain. TRF, once released from the hypothalamus, travels in the bloodstream to the anterior pituitary, where it causes the release of thyrotropin. Prolonged stimulation of the thyroid by thyroid-stimulating hormone results in an abnormal enlargement of the gland, known as goiter, a condition which has been largely eradicated by the widespread usage of iodized salt.
Thyroxine -(Pronounced As: thiroksn)
Substance secreted by the thyroid gland. The hormone thyroxine forms by combining the amino acid tyrosine with iodine. Conversely, thyroxine regulates the effect of thyrotropin by feedback inhibition, i.e., high levels of thyroxine depress the rate of thyrotropin secretion. Synthetically prepared thyroxine is used clinically in the treatment of thyroid gland deficiency diseases in adults and in the treatment of cretinism in children.
Disorders Associated With Hormones or the Lack of Hormones.
A condition due to adrenal cortical tissue insufficiency, characterised by hypotension, wasting, vomiting and muscular weakness.
Absence of menstruation.
Thyrotoxicosis, toxic goitre and Graves' disease or exophthalmic goitre.
Hyperthyroidism, a less common thyroid disorder, occurs when the thyroid gland becomes overactive and produces too much thyroid hormone. It is more prevalent among women, particularly those in their 30s and 40s. The most common form of this disorder is Graves' disease
The spectrum of possible signs and symptoms resulting from an overactive thyroid include:
- Sleep disturbances
- Muscle weakness/tremors
- Enlarged thyroid (goiter)
- Irregular menstrual periods
- Heat intolerance
- Weight loss
- Vision problems or eye irritation al activities are subject to a speeding up
Whilst the opposite condition, hypothyroidism, is evidenced by a slowing down of the body's activities.
Symptoms of overt hypothyroidism include:
- Dry, coarse skin and hair
- Mood swings
- Hoarse voice
- Difficulty swallowing
- Weight gain
- Intolerance to cold
An alteration of structure or of functional capacity due to injury or disease.
The stopping of menstruation; the period in female development when the reproductive function comes to a natural end. This is linked to a drop off in the supply of hormone secretions by the ovaries
Tortora & Grabowski (2000) Principles of Anatomy and Physiology New York John Wiley & Sons Inc.
C.T Kirkpatric (1992) Illustrated Handbook of Medical Physiology Chichester John Wiley & Sons Inc.